Spherical balls are used in numerous mechanisms, such as roll cages and roller bearings, where a low friction interface is needed between two components, one moving relative to the other. In theory, the spherical shape means that there is only a single point of contact, thus allowing the ball to roll with a very minute amount of friction. In reality, it is virtually impossible to form a perfectly spherical ball. At some level the ball is not a perfect sphere and, thus, there is additional friction occurring.
In most uses for ball bearings, the relative speed between the two components is low enough such that the deviation from a perfect spheroid associated with a conventionally formed spherical balls does not produce sufficient friction to be of concern. In high speed assemblies, however, even a small eccentricity in the spherical shape can lead to increased friction and loading.
There are various processes currently available for forming spherical balls. The type of process used will depend on the material that the ball is made from. For example, most spherical ball bearings are formed from steel or a similar malleable material. Such material can be formed using many conventional molding or forming systems, such as by pouring the molten metal into a mold and cooling the metal (hot forming). Alternately the metal can be formed into a ball by cutting stock material to a suitable length and forging the material into a spherical shape with two dies (cold forming). Both of these processes result in a ring of material or flashing remaining where the dies meet (e.g., along the circumference). As such, these processes require a deflashing stage. Deflashing can be achieved by rolling the balls between two very heavy hardened steel or cast iron parallel plates called rill plates.
It is well known that materials tumbled with an abrasive will hone preferentially on peaked areas and eventually become blunt. Although this effect is widely observed, it rarely if ever yields a true glassy or polished surface as is necessary for modern precision industrial demands.
Because of the shortfalls of tumbling, the production of precision balls is almost exclusively performed using the rill plates. For metallic material balls, the balls are squeezed into shape between the plates. For composite material balls, the balls are ground with abrasives into a spherical shape, typically requiring up to 14 weeks of continuous processing. In this latter embodiment, the abrasive material eventually wears out the precision rill plates.
Also, since the above processes require the use of malleable or molten material to form the spherical shape, the resulting ball will lack sufficient hardness. Hence, the ball is typically subjected to a heat treat step to increase its hardness. Heat treatment typically occurs after the deflashing step.
After heat treatment, the ball is cleaned and then ground or filed to achieve the required sphericity and size. Finally, the ball is typically subjected to a lapping process and then polished. Further post forming steps may be required, such as passivation, where carryover iron and other contaminants are removed and a surface film is applied to prevent atmospheric and water corrosion on the balls.
In light of the foregoing, it should be readily apparent that the formation of a spherical metallic ball is quite complicated.
If it is desired to form the spherical ball from a hard, non-metallic material, such as ceramic material, the manufacturing process is even more complex. For example, in order to form ceramic spherical balls, ceramic material is first molded into an initial shape using two mold portions. As with the metallic molding process, the ceramic molding process typically results in a band or ring of material being formed about the periphery of the ball. In order to increase the sphericity of the ball, after the molding process, since ceramic material is not malleable, the ceramic balls must be sanded by hand to eliminate the rings. This process is very time consuming and costly.
A need exists for an improved process for forming spherical components from relatively hard materials.